Switch mode power converters (SMPC) that convert AC or DC input power into DC output(s) frequently have multiple outputs. These outputs are often derived from multiple secondary windings on a single power transformer. In an SMPC, the primary winding of the power transformer is switched or commutated to the input voltage source by power switches in such a way as to provide pulses at the appropriate current and voltage levels on the secondary outputs. The DC secondary outputs are formed via rectification and subsequent filtering of the pulse train on the transformer secondaries. Each DC output voltage level depends on a turns ratio of the respective secondary windings to the primary winding as well as the ratio of the pulse width to the switching period.
The DC output voltages are then directly or indirectly regulated by a control feedback circuit. Direct regulation occurs when the feedback circuit senses at least one of the DC outputs (usually called the main output) and then modifies the switching pattern of the power switches to compensate for changes in the load or in the input voltage, thereby keeping the DC voltage level on the regulated main output relatively constant. There are many possible methods of SMPC regulation including, for instance, pulse width modulation (PWM). PWM, as a matter of fact, is one of the more widely used control and switching methods.
The SMPC feedback control usually provides quite an adequate technique of regulating the main output. In the case of PWM control, the load current variations or the input voltage variations are compensated by the feedback control loop and the proscribed output voltage level is maintained. In such an arrangement, however, the auxiliary outputs do not have the full benefits of the main feedback loop regulation. Although the main feedback loop provides some measure of regulation (cross-regulation) to the auxiliary outputs due to transformer coupling, in many cases this is inadequate and some additional techniques are needed to regulate the auxiliary outputs (post-regulation).
A typical approach to feedback control for a secondary side post regulator (SSPR) is to employ peak current-mode control (PCM) of the primary side switch of the SMPC, which permits cycle-by-cycle current limiting and overcurrent protection. Since the reflected output currents flow through the primary side switch, however, the secondary side switches must be simultaneously turned off with the primary side switch to ensure that the PCM control scheme will function properly. The simultaneous switching requirement limits the controlling of the secondary side switches to a leading-edge modulation scheme.
The use of a leading-edge modulation scheme whereby the leading-edge of the secondary side drive signal is modulated, however, introduces dynamic coupling and synchronization problems. The dynamic coupling between the secondary outputs allows perturbations or disturbances in one output to affect the other outputs. Additionally, since both trailing and leading edges of the secondary side drive signals are modulated, the primary and secondary side switches cannot be synchronized, resulting in increased electro-magnetic-interference (EMI) from the different switching frequencies.
Accordingly, there is a need in the art for an improved system and control method for secondary side post regulation that overcomes the above-described limitations.